Case Study

Composite floor
panels for military aircraft

Hexcel Composites used LUSAS Composite to analyse
honeycomb sandwich floor panels designed to support Land Rover wheel loads in military
aircraft.

Honeycomb sandwich panels are used extensively for flooring in both
military and commercial aircraft. Typically, commercial aircraft flooring is about 1cm
thick and is made of glass or carbon fibre reinforced epoxy skins with a Nomex honeycomb
core. Commercial aircraft floors are designed to withstand high compression loads,
particularly those induced by high heeled shoes. Typical floor panels weigh about 3 kg/m2.
For this military aircraft floor design, Hexcel Composites proposed using a panel with
aluminium skins and an aluminium honeycomb core simply supported between the seat rails.
The panel is required to support a Land Rover at vertical load factors of 1g and 7.8g.
With all types of finite element analyses, it is always best to start with less complex solution
methods before progressing to more advanced solutions. This case was no exception. Hexcel
Composites conducted the analysis in stages progressing from a simple linear analysis to a
full nonlinear analysis.

The panel rests on the supporting edges, and is only restrained
vertically at the bolt positions. Therefore, nonlinear joints were used along the edges to
allow the corners of the panel to "curl" up. For loads up to 7.8 g, a
geometrically nonlinear analysis was conducted to determine the maximum deflection and
load for the un-reinforced panel. For the 7.8 g load case, Hexcel Composites were also
interested in the effect of a reinforcing plate placed on top of the aircraft floor panel.
It was assumed that the reinforcing plate and floor panel would adopt the same deflected
shape. Therefore a layer of beams was used to separate the plate and panel. Beam elements
with no shear stiffness were used allowing the two panels to deflect without developing
composite action. The calculated deflection compared well with that estimated by manual
calculations. Stress levels in the honeycomb sandwich panel and reinforcing plate were
also found to be satisfactory.

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